1. Field of the Invention
The present invention relates to a two-way (or bidirectional) print apparatus and method for implementing color print by two-way scanning of a recording head for applying ink materials of plural colors in different amounts onto a print medium and, more particularly, to a two-way print apparatus and method capable of reducing uneven color occurring in two-way color print.
2. Related Background Art
In the field of the print apparatus, particularly, in the field of the print apparatus of the ink jet type, a significant subject is increase in recording speed for color print. Common techniques for increasing the recording speed include increase in recording (driving) frequency of the recording head, two-way print, etc., in addition to increase in the length of the recording head. In comparison with one-way print, the two-way print is an effective means in terms of cost as a total system, because necessary energy is dispersed on a time basis in order to gain equal throughput.
The two-way print methods, however, had a principle-based problem that uneven color occurred in a bandlike shape, because an ejection (or application) order of the ink materials of the respective colors was different in the forward direction of main scan from that in the backward direction, depending upon the recording device, particularly, depending upon the structure of the recording head. Since this problem is caused by the ejection orders of the ink materials, it appears as difference in coloring more or less where dots of different colors overlap even a little, as discussed below.
When an image is formed by applying the color materials such as pigment or dye ink materials or the like onto a print medium, an ink material of a precedently recorded dot first dyes the print medium from the surface layer to the inside of the print material. In a case whose an ink material for formation of a subsequent dot is laid in an at least partly overlapping state on the precedently recorded dot on the print medium, a large amount of ink dyes the medium in the part below the portion already dyed by the preceding ink, and thus the precedently applied ink tends to color stronger. For that reason, in the case of the conventional devices where ejection nozzles of the respective colors were arranged in the main scanning direction, since the ejection order of the ink materials in backward scanning was reverse to that in forward scanning in the two-way print, the difference of coloring caused the bandlike uneven color.
This phenomenon is not limited to only ink, but also occurs similarly with wax-based color materials and the like for process color because of the precedent-subsequent relation, though the principles are different.
Ink jet printers supporting the two-way print heretofore were constructed to avoid this problem by the following techniques.
1) To allow uneven color; or to implement the two-way print of only black (Bk).
2) To arrange the nozzles of the respective colors in the sub-scanning direction, i.e., arrange them in so-called vertical layout.
3) To provide forward nozzles and backward nozzles and switch between nozzles or heads to be used for forward scanning and for backward scanning so as to equate ejection orders of colors (refer to Japanese Patent Publication No. 03-77066).
4) To implement interlace print for rasters printed in the forward scanning and in the backward scanning, so as to complementarily cause uneven color due to difference between ejection orders at high frequency per recorded raster, thereby achieving visually uniform appearance (refer to Japanese Patent Publication No. 02-41421 and Japanese Patent Application Laid-Open No. 07-112534).
On the other hand, there is already known a technology of forming an image by combining the dots formed by different liquid droplet sizes (different liquid amounts), in order to achieve both higher image quality and a higher printing speed at the same time.
This technology allows to position the dots of different sizes within an image, and to obtain a print of higher quality with a higher speed, by forming a portion of lower granularity of the image with the relatively smaller liquid droplets and by efficiently painting a wide portion of the image with the relatively larger liquid droplets being smaller in the number of droplets.
For exploiting this technology, there have conventionally been proposed two methods, namely, in a printing apparatus equipped with a recording head capable of discharging liquid droplets of at least two sizes, or a relatively larger droplet size and a relatively smaller droplet size:    A) a method of printing with the liquid droplets of a single size selected for example according to the image resolution; and    B) a method of printing with the dots of at least two droplet sizes, in mixed manner according to the gradation data.
However, in executing the two-way print, the above prior art 1) was not an essential solution and had such a drawback that the throughput was considerably lowered in the case of inclusion of a color image. The vertical layout of 2) realized the equal ejection orders in the forward scanning and in the backward scanning, but it had such a drawback that the recording head became long and another drawback of being weak against the difference in coloring due to time difference of ejection between colors.
In the method of 3), though the recording head for forward scanning and the recording head for backward scanning were integrated on a common substrate, this was equivalent to a configuration wherein two completely different recording heads were prepared. Therefore, the method had such a drawback that there occurred the bandlike uneven color with great color difference similar to that due to the difference between heads. For example, in the case where there was difference between temperature-increasing rates of the recording heads due to difference between ratios of forward and backward data from interference with data, there was difference in ejection amounts between the recording heads, and the uneven color occurred in the bandlike shape.
The method 4) was a technique of causing regular uneven color at high frequency so as to make the uneven color unlikely to be visually perceived, and for that reason, the color difference was enhanced in certain cases because of interference, depending upon print data. For example, in a configuration where the color difference was made per raster, great color difference appeared if there existed portions with high incidence of only even rasters and portions with high incidence of only odd rasters in the forward scanning and in the backward scanning in a half-tone area such as a mesh area or the like.
Also in either of the aforementioned color printing methods A and B utilizing the different liquid droplet sizes, when the recording heads of different colors are arranged in the main scanning direction that is, they are configured with the horizontal arrangement, one-pass bidirectional printing results in a significant unevenness between the scans in two directions as in the aforementioned methods 3) and 4).